The design rule (minimum line width of patterns) of semiconductor memory devices has attained to submicron order. The g-line (light) the of high pressure mercury lamps is still used for the light source of exposing photoresists in optical lithography. In order to raise the degree of integration of silicon semiconductor devices still higher, the i-line (light) of the high pressure mercury lamps or the ultra violet light of excimer lasers will be used as light sources of optical lithography in practical levels. In further future, the optical lithography will perhaps be replaced by X-ray lithography. If the X-ray lithography is employed, the light source will be replaced by X-ray sources. Since few materials can transmit X-rays without significant absorption, entirely new X-ray lenses, resists and lithography masks will be required in order to construct a new X-ray lithography.
A lot of X-ray lithography masks have been already proposed so far. An X-ray lithography mask in general consists of a supporter, an X-ray transparent film and X-ray absorber patterns. The requirements imposed on the X-ray lithography masks are,
1 high transparency of the X-ray transparent film for X-rays and visible lights, PA1 2 flatness, PA1 3 sufficient strength, PA1 4 thinness, PA1 5 little size change induced by the change of temperature. PA1 1 Japanese Patent Laying Open NO. 57-128031 (Aug. 9, 1982) PA1 2 Japanese Patent Laying Open NO. 58-204534 (Nov. 29, 1983) PA1 3 Japanese Patent Laying Open NO. 58-135117 (Aug. 11, 1983) PA1 4 Japanese Patent Laying Open NO. 58-91100 (May 30, 1983) PA1 5 Japanese Patent Laying Open NO. 58-110494 (Jul. 1, 1983) PA1 6 Japanese Patent Laying Open NO. 1-123423 (Sep. 11, 1990) PA1 7 Japanese Patent Application NO. 1-308173 PA1 8 Japanese Patent Application NO. 1-308174
A metal mask being a thin metal plate with holes perforated according to the desired patterns has no problem regarding the transparency for X-rays. But micropatterns cannot be depicted on a metal mask because of the technical restriction of cutting. The flatness of a metal mask is also insufficient, because a thin metal plate is likely to be bent owing to the lack of rigidity. In general, a metal mask does not satisfy the requirements 2, 3, 4 and 5.
Therefore, the desired X-ray lithography masks must have the structure comprising a transparent material and X-ray absorbers deposited on the transparent material. Then, the proposed X-ray lithography masks in general consist of a thin X-ray transparent film made of the material highly transparent for X-rays, X-ray absorbers deposited thereon having high absorption for X-rays, and a supporter for sustaining these parts.
The X-ray transparent films have been made from the materials consisting of light elements. For example, boron nitride films (BN), silicon nitride films (SiN), silicon carbonate films (SiC), silicon films (Si) and diamond films (C) have already been proposed as the X-ray transparent films.
Japanese Patent Publications NO. 1-60934, and No. 1-60935, Japanese Patent Laying Opens NO. 1-227433, NO. 1-227434, and No. 1-227435 have proposed X-ray lithography masks having the X-ray transparent films made from non-diamond materials, e.g. BN, SiN, SiC, Si. Among the non-diamond materials, SiN films, SiC films, Si films are inferior to other films consisting only of light elements in X-ray transparency, because they contain silicon which has inherently not so low X-ray transparency because of its atomic weight. Further, these films have so poor strength that more than 5 .mu.m of thickness is required for practical use.
Thick films of the non-diamond materials have low transmittance also for visible light as well as for X-rays. Thus, thick films would bring about another difficulty of the mask alignment by visible light.
Diamond films are superior to the non-diamond films regarding transparency and strength. X-ray lithography masks having diamond films have been proposed by
U.S. Pat. No.--4,436,797 PA2 Japanese Patent Publication NO. 61-2632 PA2 Japanese Patent Publication NO. 59-27753 PA2 Japanese Patent Publication NO. 59-27754 PA2 EP-365366
Diamond excels in X-ray transparency, strength, and transparency also for visible light. High transparency for visible light will facilitate the mask alignment. Diamond is inherently the most promising material for X-ray transparent films.
Although many excellences of diamond had been known well, diamond films had no practical significance for a X-ray lithography mask, because diamond films could not have been synthesized till late.
However, recent development of the CVD (Chemical Vapor Deposition) methods has enabled us to grow a diamond film on an adequate substrate. The following methods 3 to 5 of synthesizing diamond films have been disclosed so far;
The advant of these new synthesizing methods has heightened the practical value of diamond films in various fields of application. The methods have facilitated to fabricate such diamond films of the shape required for X-ray transparent films.
In such a X-ray lithography mask, the part through which X-ray will transmit should be only a diamond film. The diamond film of a mask must be thin enough to reduce the absorption of X-ray energy within a satisfactory extent. If it were so thin, the diamond film would be broken spontaneously, because it could not keep its shape by itself. On the contrary, too thick diamond would weaken the transmitting X-ray by absorbing most part of energy of X-rays and would be burnt away by the heat absorbed from the X-rays.
In the case of X-ray transparent windows, not X-ray masks, we have proposed diamond films reinforced by many longitudinal and vertical crosspieces in,
Document 7 has proposed an X-ray transparent window of diamond film on which silicon parallel crosspieces are formed lengthwise and crosswise. Document 8 has proposed an X-ray transparent window of diamond film on which nickel or chromium parallel crosspieces are formed in the same manner. The X-ray window proposed by 7 is made by growing a diamond film on a silicon substrate and etching away most parts of the silicon substrate so as that lattice crosspieces and a peripheral part are reserved.
FIG. 2 shows a proposed X-ray mask having silicon crosspieces for reinforcing the diamond film. An X-ray transparent film (1) is made from diamond. This X-ray transparent film (1) is deposited on a silicon substrate (3). After the diamond film (1) is grown on the silicon substrate (3), most parts of the silicon substrate (3) are etched away except lattice parts and a periphery. Thus, lattice crosspieces and a periphery of silicon are reserved on the diamond X-ray film (1). An annular supporter (5) is fitted on the peripheral part of the silicon substrate (3). Then, X-ray absorber patterns (4), e.g. made from gold (Au) are deposited on the X-ray transparent film (1).
Besides silicon, a molybdenum substrate can be used as a substrate on which a diamond film is grown. In this case the central part of the molybdenum substrate is eliminated by etching. If some reinforcing crosspieces were left at the etching process, the crosspieces would be made of molybdenum, although such a mask having molybdenum reinforcing crosspieces has never proposed yet.
As mentioned so far, most of the current X-ray windows have no reinforcing crosspieces on the transparent film, because the transparent films are thick enough. A few current X-ray masks have reinforcing crosspieces. In these X-ray masks, the materials of the reinforcing crosspieces are exactly the same as that of the substrate, e.g. silicon (Si), molybdenum (Mo), nickel (Ni) or chromium (Cr). The reinforcing crosspieces can easily be made only by selectively etching away unnecessary parts of the substrate. Thus, the reinforcing crosspieces and the peripheral annular part of the substrate are shaped up at the same time.
Diamond has a thermal expansion coefficient smaller than that of the materials of the substrate, e.g. silicon or molybdenum on which diamond is grown. The growth of diamond generally requires the high temperature more than 600.degree. C. Since the reinforcing crosspieces are made at so high temperature that strong thermal stress would occur between the reinforcing crosspieces and the diamond film after cooling to the room temperature. The strong thermal stress is likely to distort the X-ray mask. Thus, it is difficult to produce an X-ray mask favored with flatness.
The purpose of this invention is to provide an X-ray mask which excels in X-ray transmittance, flatness and strength.